This invention relates generally to flywheels used for kinetic energy storage, and more specifically, to a fiber composite material unitary rim used in such flywheels.
The principle of the flywheel, which has been recognized for a very long time, is that a spinning wheel stores mechanical kinetic energy. Until recently, it was thought that employing flywheels to store energy for modern technological applications was out of the question because of the cost and low efficiency of energy storage as compared to flywheel weight. However, this picture has been radically changed by recent advances in materials technology and in flywheel design.
The amount of energy stored in the flywheel depends upon the mass of the rim and the angular velocity of the wheel. Energy storage varies as the square of the rotational velocity. In theory, the amount of energy that may be stored in a given flywheel may be increased indefinitely with the speed of the flywheel. However, as is well known in the art, there is a limit to the amount of energy that may be stored in a given flywheel which is dependent upon the tensile strength of the material from which the flywheel is constructed and the manner in which the various stresses created are distributed in the flywheel. For example, it has been determined that for a given flywheel weight, the best material for storing the most energy consists of a material which is of extremely low density to reduce the stresses in the wheel and which is extremely strong to withstand the stresses that are created. In so far as flywheel design is concerned, it is also known that the mass located towards the rim of the wheel contributes far greater to the energy storage than mass located towards the center of the wheel.
Flywheels have traditionally been made of metal such as high strength steel. However, because of its high density, steel is not suitable for making a flywheel capable of storing large amounts of energy for a given weight flywheel. It has been found that materials comprising a composite of fiber have much more suitable properties for flywheel construction. Such fiber composite materials are much lower in density than steel while being at least equally strong, and far stronger in some cases, than the strongest steel alloys. However, despite their superior strength, fiber composite rims are still subject to possibly destructive forces in the form of radial and hoop stress produced at extremely high rotational velocities. As a result, there have been numerous prior art attempts to produce fiber composite rims that would stand the high stresses produced, but that still permit extremely high energy storage with high rotational velocity, such as 32,000 revolutions per minute.
In an effort to overcome these problems, complex and costly fiber composite material rims have been proposed in the prior art. One example of this prior art is the multi-rim flywheel disclosed by Post in U.S. Pat. No. 3,859,868. However, this multi-rim concept requires numerous separate portions to produce a workable multiple rim, each such rim interconnected with the adjacent rim. The multi-element rim and the devices to interconnect the rims and to connect a hub or axle to the first or inner-rim, thus add greatly to the cost of production.